こそ and からこそ: emphasizing the very item or reason
Use こそ to pick out what matters against other possibilities. Distinguish emphasizing a reason with からこそ from excluding other possibilities with だけ.
This time, or right now
今度こそ合格したいです。 means that the speaker really wants to pass this time. It contrasts this opportunity with earlier ones, but does not tell us exactly how many previous failures there were.
今こそ話し合うべきです。 emphasizes that now is the important time to discuss things. Unlike 今だけ話せます。, it does not limit the possibility of talking exclusively to the present.
Returning the focus in a reply
After being thanked, こちらこそ、ありがとうございます。 says that the thanks are due from your side instead. Here こちら refers to the speaker's side of the interaction, not simply a physical location nearby.
失敗したときこそ、落ち着いて考えましょう。 means that a time of failure is precisely when we should think calmly. It does not create an exclusive rule that thinking is appropriate only after failure.
からこそ identifies the key reason
大切な友達だからこそ、本当のことを話しました。 means that it was precisely because the person was a dear friend that the speaker told the truth. Use だからこそ after a noun and forms such as 信じているからこそ after a verb.
難しいからこそ、挑戦する価値があります。 treats the difficulty itself as the reason the challenge is worthwhile. This is the speaker's evaluation of that challenge, not a universal rule that everything difficult is valuable.
Quick check: Does 今回こそ最後まで読みます。 mean the speaker will never read on any other occasion?
Show the answer and why
No. It emphasizes the determination to finish reading this time. It does not exclude every other opportunity or act of reading.